In a wireless communication system, data transmission between an access point and a user terminal occurs over a wireless channel. Depending on the system design, the same or different frequency bands may be used for the downlink and uplink. The downlink (or forward link) refers to transmission from the access point to the user terminal, and the uplink (or reverse link) refers to transmission from the user terminal to the access point. If two frequency bands are available, then the downlink and uplink may be transmitted on separate frequency bands using frequency division duplexing (FDD). If only one frequency band is available, then the downlink and uplink may share the same frequency band using time division duplexing (TDD).
To achieve high performance, it is often necessary to know the frequency response of the wireless channel. For example, the response of the downlink channel may be needed by the access point to perform spatial processing (described below) for downlink data transmission to the user terminal. The downlink channel response may be estimated by the user terminal based on a pilot transmitted by the access point. The user terminal may then send the channel estimate back to the access point for its use. For this channel estimation scheme, a pilot needs to be transmitted on the downlink and additional delays and resources are incurred to send the channel estimate back to the access point.
For a TDD system with a shared frequency band, the downlink and uplink channel responses may be assumed to be reciprocal of one another. That is, if H represents a channel response matrix from antenna array A to antenna array B, then a reciprocal channel implies that the coupling from array B to array A is given by HT, where MT denotes the transpose of matrix M. Thus, for a TDD system, the channel response for one link may be estimated based on a pilot sent on the other link. For example, the uplink channel response may be estimated based on an uplink pilot, and the transpose of the uplink channel response estimate may be used as an estimate of the downlink channel response.
However, the frequency responses of the transmit and receive chains at the access point are typically different from the frequency responses of the transmit and receive chains at the user terminal. In particular, the frequency responses of the transmit/receive chains used for uplink transmission may be different from the frequency responses of the transmit/receive chains used for downlink transmission. The “effective” downlink channel response (i.e., including the transmit/receive chains) would then be different from the reciprocal of the effective uplink channel response due to differences in the transmit/receive chains (i.e., the effective channel responses are not reciprocal). If the reciprocal of the channel response estimate obtained for one link is used for spatial processing on the other link, then any difference in the frequency responses of the transmit/receive chains would represent error that, if not determined and accounted for, may degrade performance.
There is, therefore, a need in the art for techniques to calibrate the downlink and uplink channels in a TDD communication system.